/*
 *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */

#include "common_audio/signal_processing/include/real_fft.h"

#include <stdlib.h>

#include "common_audio/signal_processing/include/signal_processing_library.h"

struct RealFFT {
    int order;
};

struct RealFFT *WebRtcSpl_CreateRealFFT(int order) {
    struct RealFFT *self = NULL;

    if (order > kMaxFFTOrder || order < 0) {
        return NULL;
    }

    self = (struct RealFFT *)malloc(sizeof(struct RealFFT));
    if (self == NULL) {
        return NULL;
    }
    self->order = order;

    return self;
}

void WebRtcSpl_FreeRealFFT(struct RealFFT *self) {
    if (self != NULL) {
        free(self);
    }
}

// The C version FFT functions (i.e. WebRtcSpl_RealForwardFFT and
// WebRtcSpl_RealInverseFFT) are real-valued FFT wrappers for complex-valued
// FFT implementation in SPL.

int WebRtcSpl_RealForwardFFT(struct RealFFT *self,
                             const int16_t *real_data_in,
                             int16_t *complex_data_out) {
    int i = 0;
    int j = 0;
    int result = 0;
    int n = 1 << self->order;
    // The complex-value FFT implementation needs a buffer to hold 2^order
    // 16-bit COMPLEX numbers, for both time and frequency data.
    int16_t complex_buffer[2 << kMaxFFTOrder];

    // Insert zeros to the imaginary parts for complex forward FFT input.
    for (i = 0, j = 0; i < n; i += 1, j += 2) {
        complex_buffer[j] = real_data_in[i];
        complex_buffer[j + 1] = 0;
    };

    WebRtcSpl_ComplexBitReverse(complex_buffer, self->order);
    result = WebRtcSpl_ComplexFFT(complex_buffer, self->order, 1);

    // For real FFT output, use only the first N + 2 elements from
    // complex forward FFT.
    memcpy(complex_data_out, complex_buffer, sizeof(int16_t) * (n + 2));

    return result;
}

int WebRtcSpl_RealInverseFFT(struct RealFFT *self,
                             const int16_t *complex_data_in,
                             int16_t *real_data_out) {
    int i = 0;
    int j = 0;
    int result = 0;
    int n = 1 << self->order;
    // Create the buffer specific to complex-valued FFT implementation.
    int16_t complex_buffer[2 << kMaxFFTOrder];

    // For n-point FFT, first copy the first n + 2 elements into complex
    // FFT, then construct the remaining n - 2 elements by real FFT's
    // conjugate-symmetric properties.
    memcpy(complex_buffer, complex_data_in, sizeof(int16_t) * (n + 2));
    for (i = n + 2; i < 2 * n; i += 2) {
        complex_buffer[i] = complex_data_in[2 * n - i];
        complex_buffer[i + 1] = -complex_data_in[2 * n - i + 1];
    }

    WebRtcSpl_ComplexBitReverse(complex_buffer, self->order);
    result = WebRtcSpl_ComplexIFFT(complex_buffer, self->order, 1);

    // Strip out the imaginary parts of the complex inverse FFT output.
    for (i = 0, j = 0; i < n; i += 1, j += 2) {
        real_data_out[i] = complex_buffer[j];
    }

    return result;
}
